He often makes mistakes---his carelessness.A、because B、becau

B because of
because of + 名词 （careless）
because 和because of 的区别：
because+ 句子,是来回答why问题,或者解释原因的；
而because of 后面直接跟一个名词、代词、动名词或者短语.它和后面的词或词组构成一个原因状语.
He often makes mistakes 【because of 】his carelessness.
翻译：他因为粗心经常犯错误.